A widely used process for preparing polystyrene-based resin foam bodies is one by which a volatile blowing agent is added to a molten resin and the mixture is kneaded under high temperature and pressure and then, after temperature adjustment, extruded through a low temperature, low pressure zone to foam the mixture and obtain an extruded foam body.
As such extruded foam bodies of polystyrene-based resins are light-weight and have excellent insulating properties as well as being relatively economical, they have found wide use as insulating materials for buildings.
Conventional production of such polystyrene-based resin foam bodies has involved the use of saturated chlorofluorocarbons (hereunder, CFCs) such as dichlorodifluoromethane (F-12), dichlorotetrafluoroethane (F-114) and trichlorofluoromethane (F-11). This is because the incombustibility and low thermal conductivity of these special freons are extremely useful for extruded foam insulating materials which must have excellent insulating properties.
However, since these special freons are very stable chemically, after they escape from the foam body they diffuse into the atmosphere and rise to the stratosphere without being degraded and result in destruction of the ozone layer, for which reason restrictions on their production and use are presently being promoted on a worldwide scale.
In place of the above-mentioned freons there have also been used hydrochlorofluorocarbons (hereunder, HCFCS) such as 1-chloro-1,1-difluoroethane (F-142b), monochlorodifluoromethane (F-22) and 1-chloro-1,2,2,2-tetrafluoroethane (F-124). Since these HCFCs have a hydrogen atom in the molecule, the lifetime of the molecule is shorter than the above-mentioned special freons and as a result they are less destructive to the ozone layer.
Nevertheless, the ozone destruction parameter (ODP) is not completely reduced to zero by including a hydrogen atom in the molecule, and thus restrictions will almost certainly be placed on their use as with special freons. Therefore, it has become an important and urgent issue to develop a blowing agent with an ODP of zero and low thermal conductivity, to replace HCFCs.
When aliphatic hydrocarbon-based blowing agents such as propane or butane have-been used in place of HCFCs, it has been impossible to obtain foam bodies with the same low thermal conductivity achieved by using the aforementioned special freons (especially F-12) or HCFCS. Furthermore, the flammability of these blowing agents has led to combustion of the resultant foam bodies, leaving a problem from the viewpoint of safety.
Here, the present inventors have made efforts to obtain extruded foam bodies with excellent insulating properties using as blowing agents hydrofluorocarbons (hereunder, HFCs), i.e. fluorocarbons with at least one hydrogen atom and containing no chlorine, which have an ozone destruction parameter of zero and excellent thermal conductivity. These contain no chlorine atoms in the molecule and thus are not ozone-destructive. Furthermore since they contain at least one hydrogen atom in the molecule, they have a short lifetime in the atmosphere and are thus considered to contribute very little to the greenhouse effect.
Despite the above-mentioned advantages of these HFCs, however, their low compatibility with polystyrene-based resins means that when large amounts of HFCs are added for more satisfactory insulating properties, under conventional production conditions, gas is released inside the die from the molten resin during the extrusion foaming which causes occurrence of foaming prior to leaving the die and thus leaves the resin torn, making it impossible to obtain aesthetically pleasing foam bodies which are uniform and have a high expansion ratio. Methods of further raising the pressure of the system in order to increase the solubility of HFCs in polystyrene have been considered. However, even when the system pressure is raised, many nuclei are generated in the die land before release to atmospheric pressure, because of the low solubility of the HFCs. The strong nucleating effect of the HFCs due to growth of the many generated nuclei in the small cells considerably lowers the air bubble size, making it impossible to obtain foams with large cross-sectional areas. As a result the density is notably increased and the desired expansion ratio cannot be achieved. In addition, the high vapor pressure of HFCs complicates molding and makes it difficult to obtain aesthetically pleasing foam bodies.
In Japanese Unexamined Patent Publication No. 4-62134 which is aimed at overcoming the aforementioned problems, there is disclosed a foaming method based on processing conditions of low temperature, high pressure and a long residence time in order to suppress the phenomenon of release of gas with low solubility in polystyrene resins.
Also, Japanese Unexamined Patent Publication No. 4-363340 proposes improving the solubility of poorly-soluble gas in polystyrene by the additional use of a cavity transfer mixer.
Nevertheless, the method disclosed in Japanese Unexamined Patent Publication No. 4-62134 still has the following unsolved problems in terms of practicality.
1) Since the foaming efficiency of the gas is lowered by carrying out the process at low temperature, ethyl chloride and/or methyl chloride, which are highly soluble in polystyrene-based resins, must be added in larger amounts than conventionally required to achieve the desired expansion ratio. Foam bodies obtained in this manner have the undesirable effect of inferior mechanical strength and dimensional stability due to plasticization of the cell membrane-forming resin by the readily soluble blowing agent.
2) Because of the great load exerted on equipment of the prior art to maintain the high pressure conditions of the process, it becomes necessary to switch to more expensive equipment with high pressure specifications, thus resulting in higher costs. Also, as the aperture of the nozzle (die) must be reduced to maintain the pressure of the system, the shear rate in the die is increased resulting in inevitable rise in frictional resistance between the molten resin and the die land surface, and it is thus difficult to obtain satisfactory skin surfaces and the surface smoothness is impaired. In addition, since the blow-up ratio must be made larger, it becomes difficult to stably obtain foam bodies with large cross-sections.
3) Costs also become higher since either the extrusion output must be lowered to lengthen the residence time or the equipment itself must be lengthened.
4) Even with optimization of the 3 processing conditions of temperature, pressure and residence time, many nuclei are generated in the die land prior to release to atmospheric pressure due to the low solubility of the HFCs. The strong nucleating effect of the HFCs due to growth of the many generated nuclei in the small cells considerably lowers the air bubble size, making it impossible to obtain foams with large cross-sections areas. As a result the density is notably increased and the desired expansion ratio cannot be achieved. In addition, the high vapor pressure of HFCs complicates molding and makes it difficult to obtain aesthetically pleasing foam bodies.
In the method in Japanese Unexamined Patent Publication No. 4-363340, there is greater loss of pressure in the equipment, and thus increasing the pressure results in greater equipment cost. Moreover, for the same reasons mentioned above, it is difficult to obtain foam bodies with high expansion rates and large cross-sections.